Known in the art are, apart from initially known hard alloys of tungsten monocarbide with cobalt (binder), hard alloys, wherein a portion of tungsten carbide is replaced with titanium, tantalum, niobium carbides. The content of tungsten carbide in these alloys is usually of from 60 to 97% by mass. Hardness of these hard alloys ranges from 86 to 92 HRA units, while their ultimate bending strength is within the range of from 20 to 90 kgf/cm.sup.2.
The most high-strength are tungsten-cobalt alloys employed for cutting iron and steel. Titanium-tungsten alloys including those containing tantalum or niobium carbide are less durable but ensure a higher resistance of a cutter and are employed mainly for cutting steel under high-speed conditions.
Recently a great attention has been paid to the use of tungsten-free hard alloys due to rather scarce sources of tungsten. As a rule, the hard base of such alloys is represented by titanium carbide, while nickel doped with molybdenum serves as a binder. These alloys have a high wear-resistance in cutting steel, but due to a high brittleness they are used mainly for semi-finish and finish operations of steel machining.
However, the machine-tool manufacture persistently demands the development of new, more wear-resistant hard alloys capable of being used for machining of hardened steel at high cutting speeds.
At the present time in the industry it is necessary to machine steels of a high hardness range of from 15 to 65 HRC units. The machining of hardened steels having hardness of from 35 to 65 HRC units is accompanied by considerable difficulties. Thus, titanium-tungsten alloys are used mainly for machining of steel having hardness not over 35 HRC units. For machining of steels with a hardness above 35 HRC units these alloys are unsuitable due to an insufficient hardness thereof.
From this standpoint the most promising are mineral-ceramic materials based on alumina Al.sub.2 O.sub.3 doped with high-melting carbides possessing a high hardness--up to 94 HRA units ("Cermets", ed. by J. R. Tincklepaud and W. B. Crandall, 1962, "Inostrannaja Literatura" (Foreign Literature) Publishing House, Moscow, p. 236-279). These materials, in fact, make it possible to carry out machining of hardened steel with a hardness of up to 65 HRC units. However, these mineral-ceramic materials have a low strength (ultimate bending strength is 70 kgf/mm.sup.2) and a low thermal conductivity, wherefore these are employed in cutting tools with a sophisticated cutter shape hindering its breaking. Despite the high hardness of mineral-ceramic materials, they cannot fully replace hard alloys in machining of steels, but only complement them in certain cutting operations.
To increase hardness of hard alloys, borides of transition metals, mainly titanium diboride, have been suggested to be added.
Thus, known is a hard alloy based on titanium diboride which consists of the following components, percent by mass:
tungsten carbide--23 to 25 PA1 cobalt--13 to 13.5 PA1 titanium diboride--the balance. PA1 titanium diboride--52 to 68 PA1 titanium carbide--13 to 17 PA1 cobalt--5 to 18 PA1 carbon--1 to 2 PA1 molybdenum and/or molybdenum boride and/or molybdenum carbide--9 to 15 PA1 titanium diboride--40 to 60 PA1 binder--3 to 30 PA1 titanium carbide--the balance;
(cf. USSR Inventor's Certificate No. 514031, Bulletin "Discoveries, Inventions, Industrial Designs and Trademarks", No. 18, published May 15, 1976, Class C 22 c 29/00).
The hard alloy having the above-specified composition is used only as an abrasive material, since it possesses no necessary mechanical strength enabling manufacture of cutters therefrom.
Known in the art is a tungsten-free hard alloy based on titanium diboride consisting of the following components, percent by mass:
(cf. USSR Inventor's Certificate No. 523954, Bulletin "Discoveries, Inventions, Industrial Designs and Trademarks" No. 29, published Aug. 5, 1976, Cl. C 22 c 29/00).
The hard alloy of the above-specified composition has a high hardness, but is unsuitable for the manufacture of cutting tools due to insufficient mechanical strength thereof and can be used only as an abrasion material.
Also known is a tungsten-free hard alloy consisting of titanium diboride, titanium carbide and a binder based on a metal of the group of iron; the components of the binder are present in the following mass proportions: B-2-3.5, Si-3.5-4.8, Ni-1, C-2, Li-0.01, Co-20 (cf. Japanese Application No. 50-20947, Tok-yo-Koho, published July 19, 1975, Cl. B 22 F 3/28).
This alloy can neither be used for machining of steel due to an insufficient mechanical strength.
Therefore, the attempts of incorporation of borides of transition metals and traditional binding metals from the group of iron in the composition of hard alloys have not resulted in the provision of durable alloys due to the formation, in these systems, of low-melting brittle eutectics of boron with metals of the group of iron or brittle borides of these metals (cf. H. J. Goldsmith, "Alloys of Implantation" part I, 1971, MIR Publishing House, Moscow, pp. 364-413).
Lack of hard alloys possessing a high wear-resistance and hardness at a sufficiently high operation durability suitable for machining of steel with a hardness of from 35 to 65 HRC units has brought about a problem of an urgent importance.
The process for the manufacture of the above-mentioned hard alloys involves the production of high-melting compounds with subsequent use of techniques of the powder metallurgy comprising preparation of a charge by intermixing of powders of the resulting high-melting compounds with a binding metal, compression of blanks and sintering at a temperature of from 1,350.degree. to 1,550.degree. C. for several hours in vacuum or hydrogen electric furnaces (cf. V. I. Tretiakov "Foundations of Physical Metallurgy and Technology of Production of Hard Alloys", 1976, Metallurgy Publishing House, Moscow, p. 7).
One of the most commonly used way of the manufacture of high-melting compounds for hard alloys (carbides, borides, nitrides of transition metals) resides in the synthesis thereof from corresponding metals (or their oxides) and non-metal (carbon, boron, nitrogen) in electric furnaces at a temperature of from 1,600.degree. to 2,200.degree. C. for a period of several hours (see the book op. cit., pp. 265-293).
Another, economically and technologically pure efficient way for the production of high-melting compounds resides in that at; least one metal selected out of IV-VI Groups of the periodic system is mixed with at least one of non-metal selected from the group of carbon, nitrogen, boron, silicon, oxygen, phosphorus, fluorine, chlorine and the resulting charge is locally ignited by any conventional method, e.g. by means of a tungsten coil. This creates a temperature necessary for initiation of an exothermal reaction of metals with non-metals in a small volume of the charge. Further the process of interaction of the charge components necessitates no use of external heating sources and proceeds at the account of the heat of the exothermal reaction per se. The reaction spontaneously propagates within the charge under burning conditions due to the heat transfer from the heated layer of the charge to the cold one at the burning speed of 4 to 16 m/sec (cf. U.S. Pat. No. 3,726,643 Cl. C01 B, published 1973).
This prior art process for the production of hard alloy involves several stages: the stage of a preliminary preparation of high-melting compounds and the subsequent treatment thereof by techniques known in powder metallurgy. Furthermore, this process is characterized by high rates of consumption of electric power.